Author's Note: This fic is for the wondrous jguda. She requested a Sam/Charlie SG-1/Numb3rs crossover, so here it is.

He knows they all think he's oblivious. That he doesn't understand people. That he's socially inept.

Well, okay, maybe the not understanding people bit is true. People aren't logical, like Larry keeps pointing out. They're irrational and unpredictable.

But he isn't oblivious. He knows Amita's in love with him - or is at least attracted. (Though he isn't exactly sure why.) He also knows he can't do anything about it. He was tempted, sure - until the Dean took him aside and "reminded" him of the rule against students and professors in romantic relationships.

(What he doesn't know is that the Dean would fight tooth and nail to keep him on his staff. A mind like Charlie's comes along once in a hundred years, and CalSci is damn lucky to have him.)

Which is why, when he arrives at Cheyenne Mountain Military Complex, he is torn between relief (military people are more predictable than most) and discomfort (academia and the military don't exactly mix).

He knows what to expect. He's done work for the military before - troop movements and stuff like that. Even if he hasn't yet done it under a few tons of mountain.

What he doesn't expect, however, is an Air Force lieutenant colonel greeting him with relief. A beautiful blonde astrophysicist lieutenant colonel.

Who, apparently, travels through wormholes in space to other planets.

And fights in a war against aliens.

Charlie doesn't know what to expect anymore.

Colorado is beautiful this time of year - or so he's heard. He hasn't actually been above-ground for five days.

He's seen Colonel Carter around the base, but hasn't really interacted with her since that first day. She'd gone off on a mission for the first two days, and came back bruised and torn. He hasn't asked why.

She seems to be well-respected, and not only because of her rank. From the headway she made on the project he was called in on, she's quite brilliant. From the rumors he's heard, she's also a quick thinker, a great shot, and a hero in her own right.

She's also beautiful even when she's covered in motor oil.

That's the first thing he notices when she comes into the lab he was allocated. Black, viscous oil all over the front of her coveralls. She's wiping her hands on a rag absently and chatting with one of the computer guys... Felger, he thinks the man's name is... who is eagerly hanging on her every word.

"Dr. Eppes." She smiles and peers over his shoulder at the various charts and graphs he has spread on the table. "Wow! I wouldn't have gotten this far in a month!"

"Well, I did have a lot of help." He notices he's tapping his pencil against the table, and stops. "And this is the only thing I'm here to do."

Felger's glaring at him, even as a short woman in glasses glares at Felger. The colonel is studying one of his graphs. She asks a question and he responds happily. He starts grabbing for relevant papers, and the colonel listens.

He's not used to people understanding him.

It's raining when Colonel Carter - "Oh, please, call me Sam" - finally wants to take a break. He knows it's raining because she asks him to go on a walk with her.

He doesn't mind. It's only a summer shower, and he'll dry. Besides, it's what umbrellas are for. Sam laughs when he asks if she wants to go back. "You call this wet?"

He asks her why she studied astrophysics. "I wanted to be an astronaut for the longest time. When I was asked to join the program, it was like a dream come true."

She asks him why he became a professor. He smiles. "I'm a mathematician. Most of us stay in academia. Teaching pays the bills."

"But you're brilliant; you could work for the NSA."

He shrugs. "I like to teach. Besides, I consult sometimes."

"And does consulting just 'pay the bills'?"

He studies her. "Sometimes it does more."

They come to a stop. She blushes. "It's, uh, stopped raining."

And it has. The sun is fighting to break through the gray storm clouds. A stray ray of light illuminates the clearing where they stand. Sam takes off the poncho she's wearing. It's purple. Why he remembers that so clearly, he's not certain.

He shakes his umbrella, and droplets go flying. Sam laughs as one splashes her nose. "Sorry about that," he says, embarrassed.

"It's not a problem." Sam looks up and smiles. "Hey, look." She points.

He's not sure what she's pointing out until he moves directly behind her. It's a rainbow. He starts thinking of angles of diffraction and wavelengths and his fingers itch for a piece of chalk.

"It's beautiful," he offers. Her smile is bright in the early afternoon sunlight.

His fingers itch for a different reason now.

Charlie's never drawn out a project like this. He's ashamed of himself at times.

Then she leans in front of him, pointing out a (purposely) missed variable, and he catches the scent of her shampoo.

The chalk squeaks on the blackboard. Sam quietly watches his progress. A familiar silence settles in his mind, and he loses himself in his work.

The chalk snaps as he pauses to collect his thoughts. He bends down to pick it up. His hand closes over Sam's.

She smiles and laughs nervously as her eyes meet his. He absently remembers that the wavelength of blue light is approximately 475 nanometers.

Then she kisses him, and he stops thinking.

Dinner is food from some diner Sam likes. He doesn't care; it's edible, and he's never been picky. Besides, he's survived on cafeteria food.

Dessert is Sam.

Romance isn't Charlie's strong point, and he knows it. It's not that he's shy or easily embarrassed - it's those social graces that Don learned so easily, and Charlie had never really picked up. Numbers are easy; people aren't. And women are even more difficult.

But he's learned to put his vast powers of concentration to good use.

Sam certainly isn't complaining.

She does seem a bit thoughtful, though.

"You know this won't last, right?" Her blue eyes are shadowed, and her voice is hoarse - from exhaustion or tears, he's not sure.

"I know." He's known the odds from the beginning. But if he hadn't played the game, he wouldn't have had a chance of winning.

"Why are you here?" He knows that tone of voice. She's unsure, more of herself than of him. He had sounded like that when he defended his thesis.

"Because you want me to be."

"You're leaving tomorrow."

"I know." Why can't he think of anything else to say?

"You know I probably won't call?"

"Yes."

"Or write?"

"Uh-huh."

"And if anything happens to me, you'll never know it?"

That makes his heart constrict, but he gets the words out anyway. "I know."

She sounds frustrated now. "So why are you here?"

"Because I want to be."

She seems to like this answer better, because she kisses him. He kisses her back.

He's never been good with words.

He's home. Don seems surprised that he's been gone for so long, but Dad just tells him to grab a shower and that dinner will be ready in half an hour.

His hair is still wet when he sits down for dinner. Sam loved his curly hair.